:root {
  --background-color: #2c3e50;
  --border-color: #7591ad;
  --nav-background-color: #34495e;
  --text-color: #34495e;
}

* {
  margin: 0;
  padding: 0;
}

body {
  width: 100vw;
  height: 100vh;
  background-color: var(--background-color);
  display: flex;
  justify-content: center;
  align-items: center;
  font-family: Shijie;
}

@font-face {
  font-family: "Shijie";
  src: url("../intro3S/汉仪正圆-45S.ttf");
}

ul {
  width: 70vw;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

li {
  list-style: none;
}

a {
  position: relative;
  color: #fff;
  text-decoration: none;
  padding: 6px 18px;
  transition: background-color 0.8s;
}

a:hover {
  background-color: orangered;
}

a::before {
  position: absolute;
  content: "";
  width: 20px;
  height: 20px;
  border: 2px solid orangered;
  border-width: 0 0 2px 2px;
  left: 0;
  bottom: 0;
  opacity: 0;
  transition:0.3s;
}

a::after {
  position: absolute;
  content: "";
  width: 20px;
  height: 20px;
  border: 2px solid orangered;
  border-width: 2px 2px 0 0;
  top: 0;
  right: 0;
  opacity: 0;
  transition:0.3s;
}

a:hover::before{
    left: -12px;
    bottom: -12px;
    opacity: 1;
}
a:hover::after{
    top: -12px;
    right: -12px;
    opacity: 1;
}